One approach to capturing images is to provide an array of solid state sensors that generate output signals in response to the intensity of light received at the individual sensors over a particular integration time. Thus, the sensors convert photon energy into electrical energy. As one possibility, the sensors may be CMOS image sensors that are arranged in a two-dimensional array to generate image information for display or storage.
A concern is that imaging devices that do not instantaneously capture all of the image information used to form a frame are susceptible to introduction of an image artifact referred to as “flicker.” When an imaged scene is illuminated by a fluorescent light source, light intensity is varied periodically in correspondence with the frequency of the alternating current (AC) power, so that flicker is introduced into the image. A fluorescent lighting system that is powered by a source of 60 Hz alternating current will exhibit periodic peaks of intensity at a rate of 120 Hz, i.e., twice the frequency of the alternating current. Thus, unless a digital imaging device includes a mechanism for addressing “beats” at this frequency, an image of a gray background captured under illumination by fluorescent lighting will include readily apparent amplitude modulations of the light intensity in a particular direction (typically the vertical direction), since the light level will vary with the capture of different lines of the image.
Techniques for preventing or removing the artifact from the imaging process are known. A common technique is to set the exposure time to a multiple of the period of the AC power waveform. With the light intensity being integrated over an integer multiple of periods of light fluctuation, the integrated light intensity is constant. For AC power waveforms of 60 Hz, the exposure time should be an integer multiple of 8.33 milliseconds.
Setting the exposure time is effective at eliminating the flicker artifact when the imaging device captures scenes which are illuminated by fluorescent lighting systems which fluctuate at the anticipated frequency. However, in many European countries, the AC power waveform has a frequency of 50 Hz, so that the flicker frequency of concern is 100 Hz. In other countries (e.g., Japan), there are multiple main frequencies (i.e., 50 Hz and 60 Hz). If a digital camera has an exposure time that is set to prevent flicker that would otherwise be introduced by illumination having a particular fluctuation period, flicker will be introduced when the digital camera is exposed to illumination having a longer or a shorter fluctuation period.
U.S. Pat. No. 6,271,884 to Chung et al. describes a digital camera that is able to adapt between use in a 50 Hz lighting environment and use in a 60 Hz lighting environment. The frame rate remains the same, but the integration time is adjustable. The integration time is defined as the amount of time that the electronic component of a particular sensor is allowed to capture light energy for each frame. When the camera is used in an environment having 60 Hz fluorescent lighting, the integration time is set at a multiple of 8.33 millliseconds, but the integration time is changed to a multiple of 10 milliseconds when the environment utilizes 50 Hz fluorescent lighting. When referring to the technique for selecting between the two integration times, the Chung et al. patent states that a typical system requires the user to set the system to define the lighting frequency or may ask the user to view an image display in which an image is generated first at one integration time setting and then at the other integration time setting, so that the user may select the best results. The patent also states that the system could detect the country in which it is operating based on system configuration data or the position of a manual switch that provides for 50 Hz versus 60 Hz lighting. As yet another alternative, the device driver can be placed into a setup mode to monitor “beats” within the intensity of data as provided by a brightness histogram block. By monitoring the beats provided by the brightness histogram block, it can be detected when the camera is being implemented with 50 Hz lighting and an integration time that is not a multiple of 10 milliseconds. The device driver can automatically adjust the integration time to a multiple of 10 milliseconds, if necessary.
Another automatic process is described by Hurwitz et al. in a publication entitled “A Miniature Imaging Module for Mobile Applications,” 2001 IEEE International Solid-State Circuits Conference (publication No. 0-7803-6608-5). The publication describes a CMOS imaging device that utilizes a conventional sensor array for forming images and utilizes additional sensors that are dedicated to the detection of flicker. Two array-height “super pixels” border the main sensor array, but are not used to form part of the main image array. The super pixels instead detect vertically averaged lighting variations. The super pixels share the lens and readout with the main array. The super pixel data is interleaved with normal pixel data during free periods within the line. A coprocessor strips out the super pixel data and performs spectral analysis using complex demodulation. Two detectors may be run in parallel, with one detector tuned to 100 Hz and the other detector tuned 120 Hz. Detected energy above a predetermined threshold indicates the presence of flicker at the tuned frequency.
U.S. Pat. No. 5,960,153 to Oster et al. describes automatic detection of the fluorescent lighting period within a particular type of electronic camera. Specifically, the automatic detection is performed within a trilinear charge coupled device (CCD) embodiment in which an image is captured by moving the camera relative to the scene of interest, rather than by holding the camera stationary. The device includes three different linear CCDs positioned side-by-side. Each of the three lines is covered by its own color filter, which typically is in the form of dye that is painted over the sensors within the line. As the camera is moved in a single pass across the image, each linear CCD records a line at a time, until the entire color image is captured. The Oster et al. patent states that to determine the fluctuation period of the lighting, the trilinear array may be maintained in a fixed position for a set period before scanning of the image. During the set period, the fluctuations of the illuminating light are detected by measurement of the integrated light intensity for different integration periods. With the trilinear array being maintained in a fixed position, the fluctuation period may be determined by any known frequency determination method, such as zero crossing determinations, Fourier transformations, and determinations of frequency content at specific frequencies.
The Oster et al. patent also describes fluctuation detection when recording an image by moving the three lines in a single pass across the image. Thus, during the pass across the image, a time-varying intensity fluctuation of the illuminating light is converted, due to the movement of the CCD, into a spatial variation of recorder light intensity. The corresponding spatial frequency may be determined by a spatial frequency analysis of the recorded image.
While the known approaches to switching the integration times on the basis of lighting operate well for their intended purposes, improvements are available. For example, systems that require dedicated sensors for detecting lighting fluctuation frequencies increase the cost of the imaging system. As another example, techniques that operate well within systems having one or more discrete linear arrays (e.g., the trilinear array) may not function as well in systems having two-dimensional arrays of cooperating sensors. What is needed is a device and method which enable cost-efficient automatic discrimination of different frequencies of lighting fluctuations during capture of an image by a two-dimensional array of sensors.